A FURTHER MICROELECTRODE STUDY OF THE INFLUENCE OF ELECTROLYTE CONCENTRATION ON THE KINETICS OF REDOX COUPLES

Steady state and fast scan voltammetry at Pt microdisc electrodes were used to investigate the Ru(NH3)6(3+)/Ru(NH3)6(2+) and IrCl6(2-)/IrCl6(3-) couples in 1 mol dm-3 chloride media as well as solutions without added electrolyte. The data were used to estimate thermodynamic and kinetic parameters for these couples and the results are compared with those for the Fe(CN)6(4-)/Fe(CN)6(3-) couple. The shifts in the formal potentials with electrolyte concentration are significant and show that more highly charged species interact more strongly with the counter-ion of the electrolyte. In the absence of electrolyte, the electron transfer reactions appear to be slower. For the Ru(NH3)6(3+)/Ru(NH3)6(2+)and IrCl6(3-) couples, the standard rate constant decreases by a factor of 2-5 (possibly a double-layer effect). This ratio supports the conclusion that the decrease of more than 100 reported for the Fe(CN)6(4-)/Fe(CN)6(3-) couple results from chemical decomposition of Fe(CN)6(4-) and Fe(CN)6(3-) in the absence of excess cations to give an inhibiting film on the electrode surface. The cations stabilize the anions by ion-ion interactions.